In the millimeter wave holographic imaging technique, the complete data information can only be obtained by performing frequency scanning over a certain frequency band so as to calculate the three dimensional image of the object. In the scanning system, the transceiving antenna is located at the topmost end and responsible for transmitting signal to the object and receiving signals reflected from the object. The requirements on the transceiving antenna that is integral with the system include: 1. the volume shall be small to facilitate integration; 2. the directivity shall be strong, with the main beam directed to the object; and 3. the frequency band is so wide to satisfy the requirement of the system on the frequency band.
In the system integration, there are series of requirements on the transceiving antenna. By taking the miniaturization, directivity and integration with the system into account, a microstrip antenna is a better choice. However, the normal microstrip antenna typically has a narrow band. If a voltage standing wave ratio <2 is taken as a criterion, the relative band is typically smaller than 10%. Taking an antenna with a center frequency 30 GHz as an example, the operating band under a voltage standing wave ratio <2 is 3 GHz. Such band is far from satisfying the usage requirements.
Usually, there are several approaches to broaden the band of a microstrip antenna, including: 1) reducing the Q value of the equivalent circuit, 2) increasing the thickness of the dielectric, decreasing the permittivity ∈r, and increasing the loss tangent tgδ, tec., which, however, will increase the loss of the antenna, 3) adding a parasitic patch or utilizing the electromagnetic coupling effect, 4) designing an impedance matching network, which, however, will increase the size of the antenna, and 5) utilizing the array technique.
The various approaches mentioned above extend the band at the cost of the increase of the volume or the reduction of the efficiency. Furthermore, the directivity diagram of the antenna will vary as a function of the specific way of extending the band.
A millimeter wave wideband antenna has been developed over the years, and the technique has been well developed. With respect to the requirement on directivity described herein, the technique that can extend the band while providing a strong directivity is rare. In the existing method of extending the band, addition of a slot in the dielectric plate or a parasitic patch is usually used, which can only meet the requirement on bandwidth, but provide a weak directivity.